A CFD Based Design Modelfor Tubular Fixed Bed CO2 Methanation Reactors under Power to Gas Operation.

Abstract

Today, there is no doubt that renewable energy will and must play a central role in the future’s energy mix. During the 2000s and 2010s the strong growth in wind and solar photovoltaic plants for electricity production was mostly prompted by economics, environmental and social drivers. However, due to recent geo-political developments (Ukraine War, South China Sea disputes), the energy independence has resurged as an additional promoter for renewable energy development. In either case, it is already demonstrated that a large participation of non-dispatchable renewable energy in the electrical grid requires seasonal and hourly storage system to balance peak and low generation periods. Power to X (PtX) technologies, allows for the required storage, transforming electrical energy surplus into other energy carriers such as methane, hydrogen and methanol. In this work, methane as final product of the PtX process is considered due to its versatility and capacity to easily been injected into the existing gas grid. Therefore, the methane storage path also known as Power to Methane (PtM) will be mostly used to describe a particular form of Power to X system, which is of interest in this study. This work addresses one of the main technical problems demanded by methanation technology: heat management and temperature control of CO2 methanation reactors. A novel Computational Fluid Dynamics based design methodology is proposed for tubular fixed bed reactors in a PtM operational context. This thesis outlines an extensive literature review regarding the current trends in reactor modelling and the operational requirements of PtM technology. The main technological challenges regarding heat management in the tubular fixed bed reactor concept have not been addressed properly. In this research a computational modelling methodology was developed to design a methanation reactor in the context of a PtM operation. A 3D transient computational fluid dynamics model was implemented in ANSYS Fluent for such purposes. First a suitable CO2 kinetic model is validated against experimental data using a simplified tubular reactor model. Then, the heat transfer process between the reactor tubes and coolant is assessed against empirical correlations used extensively in shell and tube heat exchanger design. This unique approach allows to ensure that the proposed design fulfills entirely, both product gas quality (methane content) and heat management capabilities. Then, a sensitivity study was carried out to identify relevant differences between the two most common coolant fluids used in methanation. Unlike what is found in the revised literature, the most suitable coolant in terms of pumping energy consumption and heat transfer, is thermal oil instead of molten salts. Then, an optimal flow of coolant is found, which minimizes the energy consumed in pumping, while maintaining an optimum temperature control. Finally, a design configuration is proposed based on two modules (the first one of 1 m length, and the second of 0.5 m) with interstage water condensation. Then, the design configuration obtained in the previous chapter is subjected to disruptions relevant to Power to Gas operation. Firstly, the amount of time the reactor requires to reach the steady state from a standby condition is determined. In line with similar works, the time required for the reactor to leave the warm-start condition was 330 s. As for the reactor shutdown, it took 130 s to return to its original warm-start condition. Regarding reactor feeding relevant disruptions, a 30 s H2 feed interruption prompted a transient low-temperature hot-spot. After 90 s of resuming the H2 feed, the temperature profile returned to its original values. As for temperature disruptions, a 20 K sudden inlet feed rise, promoted the formation of a lower hot-spot and a new steady state at lower temperatures. On the other hand, a 20 K feed temperature drop triggered a high temperature hot spot, which exposed the catalytic bed to its maximum operating temperature (≈923 K). The appearance of this “wrong-way” behaviour was explained by the combined effect of transient reactant concentration and thermal inertia of the bed. Temperature disruptions did not affect substantially the quality of the outlet gas, while the H2 interruption induced a 90 s latency time required to return to the minimum methane required concentration. Finally, in the absence of coolant flow or chemical reaction, the reactor maintained warm start conditions for 3 h after all heat flow ceased. The author expects that this thesis will serve as a reference tool to the designers, chemical 4 or mechanical engineering students to better understand the phenomena involved in chemical reactor engineering in the context of intermittent energy storage.